A lethally effective charm delivery system, Sandra Bullock doesn’t need to do much to win you over. That’s true even in “Our Brand Is Crisis,” a hard-working comedy in which she plays Jane, a very un-Sandra Bullock character: a mercenary political consultant trying to strategize a former Bolivian president, Castillo (Joaquim de Almeida), back into office. With his fat cigars, gilded lifestyle and assorted dead civilians cluttering his past, Castillo seems to be a very bad man, so what’s a nice girl like Jane doing in this campaign? What, for that matter, is Ms. Bullock?

Jane is outwardly there for the love of the game, for the planning and scheming, the pulse-quickening, pawn-sacrificing brinkmanship. She doesn’t seem especially interested in what Castillo stands for (the movie is only marginally more curious), and her party persuasion remains fuzzy, despite nods to her past. Like most American movies about politics, “Our Brand Is Crisis” plays it straight down the noncommittal middle. In place of an obvious party line, it has good intentions and personalities along with a faith in both American democracy and the kind of happy endings that made Mr. Smith’s trip to Washington such a success. It casts a skeptical eye at the gospel of globalization that’s preached by Jane’s team, but its outrage is more directed at cynicism than at any specific system.

The genesis of the movie is a 2005 eyepopper of a documentary, also titled “Our Brand Is Crisis.” Directed by Rachel Boynton, it focused on the efforts of a consulting group, Greenberg Carville Shrum (whose founders include James Carville), that was hired to help Gonzalo Sánchez de Lozada win the 2002 Bolivian presidential election. Granted remarkable back-room access, Ms. Boynton provided an intimate look at how politicians are packaged and sold in elections, though instructively the strategists were also true believers. “We are in this because we not only believe in democracy,” one of the consultants, Jeremy D. Rosner, said in the documentary, “but in a particular brand of democracy, which is progressive, social democratic, market-based and modern.”

The fictional version of “Our Brand Is Crisis” — it was written by Peter Straughan and directed by David Gordon Green — borrows the outline of the documentary and fills it in with a little slapstick, patches of neo-screwball banter and a throng of pleasing support players, including Anthony Mackie, Ann Dowd and Scoot McNairy. Mostly, it provides a stage for Ms. Bullock, who at first comically skulks about wearing an aggrieved grimace, a smart coat and Didion-esque sunglasses. Once Jane gets her game on, she opens up. Her arms unfold and start flapping, and her smile comes out, only to dim when an old rival, Pat Candy (Billy Bob Thornton in full charismatic slither), pops up working for another team. It’s mutual loathing at first sight, though really it’s foreplay.

With his Mr. Clean cue ball and dripping Southern honey, Pat comes across as a wildly romanticized version of Mr. Carville, with Jane suggesting some kind of attenuated version of Mary Matalin, the Republican strategist married to Mr. Carville. (Mr. Thornton played a Carville-type figure in “Primary Colors.”) Ms. Matalin didn’t play a public role in the 2002 Bolivian election and she’s not in the documentary, but the filmmakers and their stars have fun with the idea that politics is sex for those who want to keep their clothes on. It’s cute for a while. The stars are pros, and their scenes, often staged so that the characters are within breathing distance of each other, have snap. In moments, Mr. Green seems to be trying to turn this into a Katharine Hepburn and Spencer Tracy romance.

That doesn’t happen, partly because the filmmakers have a morality lesson to finish selling. That kills the movie, but the filmmakers don’t approve of Jane the cynic and don’t want you on her side. The problem is that they and Ms. Bullock do such a good job selling this Jane that they do exactly what they’re supposed to do: They make you curious about her. You want to know how the Janes of the world happen. Is it money? Power? Ideology? You never find out. Instead, Jane starts palling around with a young Bolivian idealist with leaky eyes, Eddie (Reynaldo Pacheco), which leads to smiles, camaraderie and a visit to a slum for some rueful laughs about Adam Smith and the invisible hand. Soon her eyes are shining, too, and this character study is selling out that character as fast as it can.